1. Field of the Invention
The present invention relates to a pattern formation resist and a pattern formation method using the same and, more particularly, to a pattern formation resist exposed with deep UV and a pattern formation method using the same.
2. Description of the Related Art
Pattern formation resists are widely used in the field of electronic parts, such as semiconductor integrated circuits, which require various microprocessing techniques. In particular, it is required to form fine patterns using the resists of this type because users desire multiple functions and high packing densities in electronic equipment. As an exposure apparatus for use in this pattern formation, a step-and-repeat type reduction-projecting mask aligner is known. Examples of a radiation used in this exposure apparatus are g line (wavelength 436 nm), h line (wavelength=405 nm), and i line (wavelength=365 nm) of a mercury lamp, and XeF (wavelength=351 nm), XeCl (wavelength=308 nm), KrF (wavelength=248 nm), KrCl (wavelength=222 nm), ArF (wavelength=193 nm), and F2 (wavelength=157 nm) as an excimer laser. In order to form fine patterns, the wavelength of a radiation is preferably as short as possible. For this reason, a demand has arisen for a resist which is exposed with deep UV such as an excimer laser.
Conventionally known examples of the resist for an excimer laser are a resist consisting of an acryl polymer, such as polymethylmethacrylate (PMMA) or polyglutarmaleimide (PGMI); and a resist consisting of a polymer, in which phenol is bonded in a molecule, and an azide photosensitive agent. However, the former resist is low in sensitivity with respect to an excimer laser and poor in dry etching resistance. Although the latter resist has a high sensitivity and a high dry etching resistance, the shape of a pattern formed by this resist is a reversed triangle. Therefore, it is difficult to control exposure and development steps.
Recently, U.S. Pat. Nos. 4,491,628 and 460,310, for example, disclose resists each consisting of a polymer, in which a group unstable against acids is substituted on the side chain of a resin having a dry etching resistance, and a compound which produces an acid when irradiated with an ionizing radiation. However, since the group unstable against acids is substituted on the side chain of the polymer in these resists, no stable sensitivity nor stable resolution can be obtained.
From this point of view, Published Unexamined Japanese Patent Application No. 64-35433 describes a resist containing a binder consisting of an alkali-soluble polymer, and an organic compound having a group which forms a strong acid by a radiation effect together with a group which decomposes into an acid. An onium salt is exemplified as the organic compound in Published Unexamined Japanese Patent Application No. 64-35433, and an iodonium salt and a sulfonium salt are disclosed as practical examples of the onium salt. However, since the onium salt decomposes during storage, sensitivity stability upon exposure is hindered. In addition, the onium salt largely absorbs light having a wavelength of 248 nm. Therefore, if a large amount of the onium salt is used, resolution is lowered. Consequently, it is impossible to add a sufficient amount of the onium salt, in consideration of the above relationship between the onium salt and the resolution. For this reason, the solubility of unexposed portions cannot be satisfactorily suppressed in a development step subsequent to exposure, and this makes it difficult to form fine patterns.
On the other hand, other problems arise in various exposure methods along with reduction in minimum dimensions. For example, in exposure using light, an interference occurs by reflected light components due to steps formed on a substrate (e.g., a semiconductor substrate) and largely influences dimensional precision. In electron beam exposure, on the other hand, when a resist is micropatterned, it is impossible to increase the ratio of the height to width of the pattern due to a proximity effect caused by backscattering of electrons.
As a method of solving the above problems, a multilayered resist process has been developed, and a summary of this process is described in "Solid State Technology," 74. 1981. In addition, many studies concerning this multilayered resist process have been reported. A method which is presently, most generally attempted is a three-layered structure resist process. More specifically, this three-layered structure includes a lowest layer which has functions of flattening steps on a semiconductor substrate and preventing reflection from the substrate, an interlayer which serves as an etching mask of the lowest layer, and an uppermost layer as a photosensitive layer.
The above three-layered resist process has an advantage that finer patterning can be performed than in the case of a single-layered resist process. However, according to this three-layered resist process, the number of process steps before pattern formation is undesirably increased. That is, no resist can satisfy both photosensitivity with respect to a radiation, such as deep UV, and resistance against reactive ion etching using an oxygen plasma. Therefore, these functions must be separately imparted to different layers to result in a three-layered structure. As a result, the number of process steps is increased by those required for the layer formation.